Messages in this thread | | | From | "Huang\, Ying" <> | Subject | Re: [RFC] llist: Fix code comments about llist_del_first locking | Date | Fri, 09 Dec 2016 10:26:41 +0800 |
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Joel Fernandes <joelaf@google.com> writes:
> On Thu, Dec 8, 2016 at 6:12 PM, Huang, Ying <ying.huang@intel.com> wrote: >> Joel Fernandes <joelaf@google.com> writes: >> >>> On Thu, Dec 8, 2016 at 4:42 PM, Joel Fernandes <joelaf@google.com> wrote: >>>> On Thu, Dec 8, 2016 at 4:35 PM, Huang, Ying <ying.huang@intel.com> wrote: >>>>> Joel Fernandes <joelaf@google.com> writes: >>>>> >>>>>> Usage llist_del_first needs lock protection, however the table in the >>>>>> comments of llist.h show a '-'. Correct this, and also add better >>>>>> comments on top. >>>>>> >>>>>> Cc: Huang Ying <ying.huang@intel.com> >>>>>> Cc: Ingo Molnar <mingo@kernel.org> >>>>>> Cc: Will Deacon <will.deacon@arm.com> >>>>>> Cc: Paul McKenney <paulmck@linux.vnet.ibm.com> >>>>>> Signed-off-by: Joel Fernandes <joelaf@google.com> >>>>>> --- >>>>>> include/linux/llist.h | 19 ++++++++++--------- >>>>>> 1 file changed, 10 insertions(+), 9 deletions(-) >>>>>> >>>>>> diff --git a/include/linux/llist.h b/include/linux/llist.h >>>>>> index fd4ca0b..15e4949 100644 >>>>>> --- a/include/linux/llist.h >>>>>> +++ b/include/linux/llist.h >>>>>> @@ -3,14 +3,15 @@ >>>>>> /* >>>>>> * Lock-less NULL terminated single linked list >>>>>> * >>>>>> - * If there are multiple producers and multiple consumers, llist_add >>>>>> - * can be used in producers and llist_del_all can be used in >>>>>> - * consumers. They can work simultaneously without lock. But >>>>>> - * llist_del_first can not be used here. Because llist_del_first >>>>>> - * depends on list->first->next does not changed if list->first is not >>>>>> - * changed during its operation, but llist_del_first, llist_add, >>>>>> - * llist_add (or llist_del_all, llist_add, llist_add) sequence in >>>>>> - * another consumer may violate that. >>>>>> + * If there are multiple producers and multiple consumers, llist_add can be >>>>>> + * used in producers and llist_del_all can be used in consumers. They can work >>>>>> + * simultaneously without lock. But llist_del_first will need to use a lock >>>>>> + * with any other operation (ABA problem). This is because llist_del_first >>>>>> + * depends on list->first->next not changing but there's no way to be sure >>>>>> + * about that and the cmpxchg in llist_del_first may succeed if list->first is >>>>>> + * the same after concurrent operations. For example, a llist_del_first, >>>>>> + * llist_add, llist_add (or llist_del_all, llist_add, llist_add) sequence in >>>>>> + * another consumer may cause violations. >>>>>> * >>>>>> * If there are multiple producers and one consumer, llist_add can be >>>>>> * used in producers and llist_del_all or llist_del_first can be used >>>>>> @@ -19,7 +20,7 @@ >>>>>> * This can be summarized as follow: >>>>>> * >>>>>> * | add | del_first | del_all >>>>>> - * add | - | - | - >>>>>> + * add | - | L | - >>>>> >>>>> If there are only one consumer which only calls llist_del_first(), lock >>>>> is unnecessary. So '-' is shown here originally. But if there are >>>>> multiple consumers which call llist_del_first() or llist_del_all(), lock >>>>> is needed. >>>> >>>> I think this needs to be made more clear in the table. The table >>>> doesn't clear say whether it describes the preceding paragraph >>>> (multiple producers and one consumer), or if it describes the multiple >>>> producers and one consumer case. So either we should have 2 tables, or >>> >>> Sorry, I meant "or if it describes the multiple producer and multiple >>> consumer case". >> >> I tried to describe both cases in the original table. >> >> * | add | del_first | del_all >> * add | - | - | - >> * del_first | | L | L >> * del_all | | | - >> >> The 'L' for "del_first * del_first" means multiple consumers uses >> llist_del_first() need lock. And the 'L' for 'del_first * del_all' >> means multiple consumers uses llist_del_first() and llist_del_all() need >> lock. > > Ok, now I get it - so basically the table describes one > producer/consumer vs another producer/consumer, in other words you are > just describing contention between any 2 operations. Thanks for > clarifying. I will respin the comments to explain this a bit better if > that's Ok with you.
It is good for me to improve the comments.
Best Regards, Huang, Ying
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